Flame detection system

ABSTRACT

A flame detector, particularly for use in effecting automatic control of the fuel supply to burners, has a capacitor connected to an electrode which is disposed in the flame, when present at one of the burners or at a pilot burner, so that the capacitor charges up through the rectifying action of a said flame when an alternating current is supplied to the electrode.

United States Patent inventor Kenneth R. Wade 5 R fe e Ci d Burscough, Ill- Ormskirk, England UNITED STATES PATENTS all: 2 3 1969 2,410,524 11/1946 Richardson etal. 431/25 Patemed f (1971 2,43l,l58 11/1947 Yates 328/6 9 Assignee United Gas Industries Limited Emi i mom a 2 3,437,884 4/1969 Mandock 61811.. 325/6 x y G Bimi 2,297,821 10/1942 Whempner.... 340/228.l 32 2 3,202,976 8/1965 Rowell 340 228.1 3,238,423 3/1966 Giuffrida 328/6 X Primary Examiner-Carroll B. Dority, Jr. Attorney- Larson and Taylor g g T p SYSTEM ABSTRACT: A flame detector, particularly for use in effect- 'awmg ing automatic control of the fuel supply to b rn r h a US. Cl 431/25, capacitor connected to an electrode which is disposed in the 328/6, 431/48, 431/50, 43 N78 flame, when present at one of the burners or at a pilot burner, Int. Cl F23n so that the capacitor charges up through the rectifying action Field 0! Search 431/78, 25, of a said flame when an alternating current is supplied to the 48, 328/6; 340/228.l electrode.

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SUPPLY I2 23 24 COMMON (:flRTH +l0vDc FLAME DETECTION SYSTEM This invention relates to systems for the detection of flames, and particularly, but not exclusively, the detection of flames in gas heating installations.

There is present, in all types of flame, a certain amount of ionization. As a consequence of the different mobilities of the ions formed in the relatively high and low temperature regions of the flame there is in general a space charge distribution in the flame, the effect of which may be observed as a difference in electrical potential between relatively highand low-temperature regions of the flame. This potential difference may typically be of the order of 20 millivolts for a small methane flame, and may be measured by inserting two electrodes into different regions of the flame and measuring the potential difference therebetween by means of a high-impedance voltmeter. If the electrodes in the flame were connected to an electrical supply rather than to a voltmeter, it would be observed that a larger current flowwould take place between the electrodes for one polarity of the supply than for the opposite polarity thereof. That is, the flame acts in part as a rectifier or diode. The ratio between the effective resistance between two points in the flame to current flow in opposite respective directions typically ranges from 1:2 to 1:10, depending on the positions of the two points and the flame characteristics: the lower resistance of the flame, that is, its resistance in the forward" direction of the effective diode, is typically of the order of 50 megohms.

The present invention makes use of the above-described rectifying action characteristic of a flame to detect the presence of a flame.

Accordingly the present invention provides a flame detector in which the presence of a flame is detected by means responsive to a charge developed on a capacitor from an alternating current source through the rectifying action of said flame.

Preferably the means responsive to the charge on said capacitor is effective to prevent the supply of fuel to said flame, and/or to another flame, upon extinction or partial extinction of said flame.

Where several burners are supplied from a common fuel supply, flame detectors according to the invention may be utilized to control the common fuel supply so as to shut off this supply in the event of any one of the flames being extinguished.

Thus the invention also provides a flame detection system including a plurality of said flame detectors, and means gating the outputs of the flame detectors together, said means being responsive both to the presence or absence of flames at all the detectors collectively and to the presence or absence of a flame at any one detector. Said gating means preferably include a first relay which is energized when a signal indicating the presence of a flame is received from any one flame detector, and a second relay which is energized when signals indicating the presence of a flame are received from all the flame detectors. The system may include a controller for controlling the supply of fuel to burners with which said flame detectors are associated, said controller being operatively associated with said relays so that fuel can be supplied to the burners for ignition only when both said relays are initially deenergized.

The present invention is of particular use for controlling the gas supply to large industrial gas burners. Here, a main gas feedpipe may supply several small gas burners which collectively form the overall burner installation. The method used to ignite the small burners has previously been to provide a pilot flame for lighting each of the small burners. One difficulty with this arrangement is that, should the supply of gas from the main gas feedpipe become temporarily disconnected and the small burners and their associated pilot flames extinguished, but subsequently the gas supply restored, there exists a danger of a gas explosion.

The flame detection system of the present invention allows the flames of individual burners to be continuously monitored, thus substantially reducing the risk of a gas explosion in the above-described circumstances.

The invention is further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram showing means responsive to a charge developed on a capacitor, forming a simple flame detector according to the invention;

FIG. 2 is a circuit diagram of a flame detector according to a preferred embodiment of the invention, and

FIG. 3 is a schematic diagram which shows how a typical flame detection system including flame detectors according to the invention is used for controlling the supply of fuel to a series of burners.

FIG. I shows a fuel burner 13 substantially of metal having an electrode 11 arranged adjacent the burner so that one end of the electrode just enters a flame 10 formed at the burner 13.

A capacitor 15 has one plate connected through a resistor 16 to the electrode 11 and its other plate connected to the metal burner 13 via a connection 12. It is arranged for an alternating voltage to be applied between the electrode 11 and the connection 12 through a high-impedance direct-current blocking capacitor 14.

The capacitor 15 is connected across the input terminals of a respective high-input impedance low-output impedance 17 through a resistor 18.

The amplifier 17 in the embodiments of the invention illustrated in FIGS. I and 2 comprises a single transistor 19 connected in the common collector or emitter follower mode having a load resistor 20 in its emitter lead from across which the amplifier output is taken. This arrangement constitutes a simple flame detector according to the invention. In operation of this flame detector an alternating voltage, for example volts AC, is applied through the capacitor 14 between the electrode 11 and the connection 12. When a flame 10 is present at the burner 13 the rectifying action of the flame causes the capacitor 15 to be charged up through the resistor 16. The potential difference across the capacitor 15 is applied to the input of the amplifier 17 through the resistor 18. A direct voltage which is proportional to the potential difference across the capacitor 15 is developed across the load resistor 20 of the transistor 19 and can be used to provide an indication of the presence or absence of a flame at the burner 13.

FIG. 2 shows a flame detector 5 according to the invention in which the output 21 from the transistor 19 of the amplifier 17 is fed to the base of a complementary transistor 22. It is arranged that, with no flame present, the transistor 22 is biased into its conducting state by virtue of its base being connected to a supply which is more positive than that on its emitter. The collector and emitter of transistor 22 are connected to the positive (+10 volts) and negative (-10 volts) direct-current supply lines respectively, via resistors 38 and 39 respectively. The values of the resistors 38 and 39 are chosen such that when transistor 22 is fully conducting its collector 23 is at a voltage which is negative with respect to earth.

When a flame 10 is present direct current flows from the base of transistor 19, causing a negative charge to appear at said base. This causes transistor 19 to conduct and hence its emitter 21 to become more negative.

This has the effect of reducing the conduction of the transistor 22 and in consequence the potential of the collector 23 of transistor 22 will become more positive. With a suffciently strong flame the transistor 19 will become fully conducting and the transistor 22 will be completely nonconductmg.

It will thus be seen that the collector 23 of the transistor 22 will be at a negative potential with respect to earth with no flame present and at a positive potential with respect to earth when a flame is present.

A further transistor 24 operating in the emitter follower mode has its base connected to the collector 23 of transistor 22 and its emitter 25 connected to the negative supply line (-10 volts) via a resistor 28. The emitter 25 of transistor 24 will be at substantially the same potential as its base. Thus when no flame is present the emitter 25 of transistor 24 will be at a negative potential with respect to earth and when a flame is present the emitter 25 will be at a positive potential with respect to earth.

When it is required that several flames should be detected a respective detector 5 is provided for each burner and the emitter 25 of each transistor 24 is connected via a diode 26 to an OR-line 29 and via a diode to an AND-line 30.

A flame present" indication on a detector 5 is given by the respective emitter 25 being at a positive potential. In this condition diode 26 is conducting and current will flow through the R line into the trigger 31 of a thyristor 32 common to all the detectors 5, causing the thyristor 32 to conduct. The thyristor 32 is connected in series with a first relay 33 which is energized when the thyristor 32 conducts, in response to a flame present" signal. Thus it will be seen that the presence of any one flame, or the existence of a fault in any detector which simulates a flame, will cause the relay 33 to be energized.

The AND-line 30 is connected to the trigger 35 of a thyristor 36, which is in turn connected to a positive direct current supply via a resistor 34. When no flame is present emitter 25 of transistor 24 is at a negative potential and the diode 27 is conducting. The trigger 35 of the thyristor 36 is therefore also at a negative potential; hence the thyristor 36 is not conducting. If, however, a flame is present at each burner the emitter 25 of every detector 5 is at a positive potential and the diode 27 of every detector 5 is not conducting. Consequently the trigger 35 of the thyristor 36 becomes positive, causing the latter to conduct. A second relay 37 is connected in series with the thyristor 36 and is energized when the thyristor 36 conducts.

it will thus be seen that, where several detectors 5 are connected to the ORline 29 and AND-line 30, if any one detector is giving a flame present" signal the relay 33 will be energized and if all the detectors 5 are giving a flame present" signal the second relay 37 will be energized.

In FIG. 3 there is shown a controller 6 which incorporates the relays 33 and 37 and their associated energizing circuits. Also incorporated in thecontroller 6 is a means for energizing an electrical actuator 7' of a fuel valve 9 included in a fuel supply line 8 leading to all the burners. The controller 6 is such that, prior to ignition, the actuator 7 can only be energized to open the valve 9 if both relays 33 and 37 are not energized. Thus should any one of the flame detectors 5 provide a flame present signal when the valve 9 is in fact closed prior to ignition, the controller 6 will prevent opening of the valve, indicating a fault in the integrity of the system.

The controller 6 is also such that, following ignition, both relays 33 and 37 must be energized, indicating that all burners are lit, before the system can be left unattended with the valve 9 maintained open.

The extinction of the flame on any one burner will cause the second relay 37 to become deenergized, causing deenergization of the actuator 7 to close the valve 9, shutting oi? the fuel supply to all the burners automatically.

lclaim:

1. A flame detector comprising electrode means positioned at the site of a flame, a capacitor connected to said electrode means, means for supplying an alternating current to said electrode means, and means responsive to the presence of charge developed on said capacitor from said alternating current through the rectifying action of a flame when present at said site for producing a first output on a first output line which represents flame present and responsive to the absence of charge developed on said capacitor for producing a second output on a second output line which represents "flame absent, said means responsive to the presence and absence of charge on said capacitor including a first transistor having a base connected to said capacitor such that said first transistor becomes conducting when a flame is present. a second transistor having a base connected to the emitter of the first transistor such that the second transistor is turned off when a flame is present, a third transistor having a base connected to the collector of the second transistor such that the emitter of said third transistor is at a positive potential when a flame is present and at a negative potential when no flame 18 present,

and first means connected between said third transistor emitter and one of the output lines which operates to conduct only when said emitter is positive and second means connected between said third transistor emitter and the other of the output lines which operates to conduct only when said emitter is negative.

2. A flame detector as claimed in claim 1, wherein said first and second means are diodes.

3. Flame detection system including a plurality of flame detectors, each comprising electrode means positioned at a respective flame site, a capacitor connected to said electrode means, means supplying an alternating current to said electrode means and means responsive to charge developed on said capacitor from said alternating current through the rectifying action of a flame for detecting the presence or absence of a flame at said site and for providing one or the other of two output signals representing respectively the presence of said flame and the absence of said flame; and controller means, responsive both to the flame present signals and to the flame absent signals, for producing a burner-enabling output.

4. Flame detection system as claimed in claim 3 which said controller means includes means for gating all of said flame present" signals together and applying said flame present" signals to operate a first relay when any one such signal is present and means for gating all of said flame absent signals together and applying said flame absent" signals to operate a second relay when any one flame absent signal is present, and wherein said burner-enabling .output is produced only when both said first and second relays are operated.

5. Flame detection system as claimed in claim 4, wherein said first relay is energized by the presence of any one of said flame present" signals and said second relay is deenergized by the presence of any one of said flame absent signals.

6. Flame detection system as claimed in claim 5, said controller means further comprises a fuel supply valve supplying fuel to the said flame sites, said controller being controlled by said relays and being effective to open said valve initially only when both relays are initially deenergized.

7. Flame detection system as claimed in claim 6, in which the controller means is effective, after ignition of fuel has commenced, to maintain the fuel supply for unattended operation thereof only when both said relays are energized.

8. A flame detector comprising electrode means positioned at the site of a flame, a capacitor connected to said electrode means, means for supplying an alternating current to said electrode means and a solid-state circuit means responsive to the presence of charge developed on said capacitor from said alternating current through the rectifying action of a flame when present at said site for producing a first output on a first output line which represents flame present and responsive to the absence of charge developed on said capacitor for producing a second output on a second output line which represents flame absent," said solid-state circuit means including at least one transistor the output of which is positive or negative according to whether or not a flame is present and a pair of diodes connecting the output of said transistor to one or other of the two output lines according to whether the said output is positive or negative. 

1. A flame detector comprising electrode means positioned at the site of a flame, a capacitor connected to said electrode means, means for supplying an alternating current to said electrode means, and means responsive to the presence of charge developed on said capacitor from said alternating current through the rectifying action of a flame when present at said site for producing a first output on a first output line which represents ''''flame present'''' and responsive to the absence of charge developed on said capacitor for producing a second output on a second output line which represents ''''flame absent,'''' said means responsive to the presence and absence of charge on said capacitor including a first transistor having a base connected to said capacitor such that said first transistor becomes conducting when a flame is present, a second transistor having a base connected to the emitter of the first transistor such that the second transistor is turned off when a flame is present, a third transistor having a base connected to the collector of the second transistor such that the emitter of said third transistor is at a positive potential when a flame is present and at a negative potential when no flame is present, and first means connected between said third transistor emitter and one of the output lines which operates to conduct only when said emitter is positive and second means connected between said third transistor emitter and the other of the output lines which operates to conduct only when said emitter is negative.
 2. A flame detector as claimed in claim 1, wherein said first and second means are diodes.
 3. Flame detection system including a plurality of flame detectors, each comprising electrode means positioned at a respective flame site, a capacitor connected to said electrode means, means supplying an alternating current to said electrode means and means responsive to charge developed on said capacitor from said alternating current through the rectifying action of a flame for detecting the presence or absence of a flame at said site and for providing one or the other of two output signals representing respectively the presence of said flame and the absence of said flame; and controller means, responsive both to the flame present signals and to the flame absent signals, for producing a burner-enabling output.
 4. Flame detection system as claimed in claim 3 which said controller means includes means for gating all of said ''''flame present'''' signals together and applying said ''''flame present'''' signals to operate a first relay when any one such signal is present and means for gating all of said ''''flame absent'''' signals together and applying said ''''flame absent'''' signals to operate a second relay when any one ''''flame absent'''' signal is present, and wherein said burner-enabling output is produced only when both said first and second relays are operated.
 5. Flame detection system as claimed in claim 4, wherein said first relay is energized by the presence of any one of said ''''flame present'''' signals and said second relay is deenergized by the presence of any one of said ''''flame absent'''' signals.
 6. Flame detection system as claimed in claim 5, said controller means furtHer comprises a fuel supply valve supplying fuel to the said flame sites, said controller being controlled by said relays and being effective to open said valve initially only when both relays are initially deenergized.
 7. Flame detection system as claimed in claim 6, in which the controller means is effective, after ignition of fuel has commenced, to maintain the fuel supply for unattended operation thereof only when both said relays are energized.
 8. A flame detector comprising electrode means positioned at the site of a flame, a capacitor connected to said electrode means, means for supplying an alternating current to said electrode means and a solid-state circuit means responsive to the presence of charge developed on said capacitor from said alternating current through the rectifying action of a flame when present at said site for producing a first output on a first output line which represents ''''flame present'''' and responsive to the absence of charge developed on said capacitor for producing a second output on a second output line which represents ''''flame absent, '''' said solid-state circuit means including at least one transistor the output of which is positive or negative according to whether or not a flame is present and a pair of diodes connecting the output of said transistor to one or other of the two output lines according to whether the said output is positive or negative. 